Method for transmitting a data signal in a MIMO system

ABSTRACT

A method for transmitting a data signal by a transmission unit of a wireless multiple-input/multiple-output (MIMO) communication system. The communication system includes the transmission unit and a reception unit, the transmission unit having a plurality of transmission antennas and the reception unit having a plurality of reception antennas. The method includes performing a first transmission of a data signal, the first transmission including transmitting the data signal by each one of the plurality of transmission antennas, and performing a second transmission of the data signal at a time later than the first transmission, the second transmission including transmitting at least one spectrally modified signal variant of the data signal by at least one antenna of the plurality of transmission antennas.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/813,169 filed on Jun. 10, 2010.

FIELD

The present invention relates to a method for transmitting a data signalin a wireless multiple-input/multiple-output (MIMO) communicationsystem, a method of receiving a data signal in a MIMO communicationsystem, a transmission unit to transmit a data signal in a MIMOcommunication system, and a reception unit to receive a data signal in aMIMO communication system.

BACKGROUND

Radio communication performance can be increased by use of multipleantennas and multiple-input/multiple-output (MIMO) techniques. The datathroughput and link range can be increased by employing MIMO techniqueswithout additional bandwidth or transmit power. In such MIMO systems,spatial division multiplexing (SDM) is one technique used in whichdifferent spatial streams of data are sent from each transmit antenna.Since these streams carry different data, the overall data rate of thesystem is increased. A further improvement of the radio communicationperformance is to be expected when combining the above techniques withmulti-carrier transmission and reception methods like, for example, thewell-known orthogonal frequency division multiplex (OFDM) transmissionand reception techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of embodiments and are incorporated in and constitute apart of this specification. The drawings illustrate embodiments andtogether with the description serve to explain principles ofembodiments. Other embodiments and many of the intended advantages ofembodiments will be readily appreciated as they become better understoodby reference to the following detailed description. Like referencenumerals designate corresponding similar parts.

FIG. 1 shows a flow diagram of a method for transmitting a data signalin a MIMO communication system according to an embodiment;

FIG. 2 shows a flow diagram of a method for transmitting a data signalin a MIMO communication system according to an embodiment;

FIG. 3 shows a flow diagram of a method for receiving a data signal in aMIMO communication system according to an embodiment;

FIG. 4 shows a schematic block representation of a wireless MIMO systemperforming an embodiment of a method as depicted in FIG. 1 or 2.

FIG. 5 shows a schematic block representation of a wireless MIMO systemperforming an embodiment of the method depicted in FIG. 3.

FIG. 6 shows a diagram to illustrate the introduction of frequencydiversity with the second transmission;

FIG. 7 shows a diagram to illustrate the introduction of frequencydiversity, including a frequency shifted version of the sequence usedfor the second transmission;

FIG. 8 shows a schematic block representation of a wireless MIMO systemaccording to an embodiment;

FIG. 9 shows a schematic block representation of a transmission unitaccording to an embodiment; and

FIG. 10 shows a schematic block representation of a reception unitaccording to an embodiment.

DETAILED DESCRIPTION

The aspects and embodiments are described with reference to thedrawings, wherein like reference numerals are generally utilized torefer to like elements throughout. In the following description, forpurposes of explanation, numerous specific details are set forth inorder to provide a thorough understanding of one or more aspects of theembodiments. It may be evident, however, to one skilled in the art thatone or more aspects of the embodiments may be practiced with a lesserdegree of the specific details. In other instances, known structures andelements are shown in schematic form in order to facilitate describingone or more aspects of the embodiments. It is to be understood thatother embodiments may be utilized and structural or logical changes maybe made without departing from the scope of the present invention.

In addition, while a particular feature or aspect of an embodiment maybe disclosed with respect to only one of several implementations, suchfeature or aspect may be combined with one or more other features oraspects of the other implementations as may be desired and advantageousfor any given or particular application. Furthermore, to the extent thatthe terms “include”, “have”, “with” or other variants thereof are usedin either the detailed description or the claims, such terms areintended to be inclusive in a manner similar to the term “comprise”. Theterms “coupled” and “connected”, along with derivatives may be used. Itshould be understood that these terms may be used to indicate that twoelements co-operate or interact with each other regardless of whetherthey are in direct physical or electrical contact, or they are not indirect contact with each other. Also, the term “exemplary” is merelymeant as an example, rather than the best or optimal. The followingdetailed description, therefore, is not to be taken in a limiting sense,and the scope of the present invention is defined by the appendedclaims.

The apparatuses and methods as described herein are utilized as part ofand for radio transmission systems, in particular for systems operatingin the Orthogonal Frequency Division Multiplex (OFDM) mode. Theapparatuses disclosed may be embodied in baseband segments of devicesused for the transmission or reception of data signals such as e.g. OFDMradio signals, in particular transmitters like base stations or relaystations and receivers like mobile phones, hand-held devices or otherkinds of mobile radio receivers. The described apparatuses may beemployed to perform methods as disclosed herein, although those methodsmay be performed in any other way as well.

An OFDM communication link may be operable with an amount of Nsub-carriers with N being 2048, for example. Sub-carriers of such OFDMtransmission systems may comprise a single frequency each. They may alsocomprise a plurality of frequencies, for example, adjoining frequenciesin a frequency range or any arbitrary sub-set of frequencies. The numberof frequencies included in a sub-carrier may, in particular, not belimited to any number of frequencies.

The methods and units as described herein are utilized withinMultiple-Input/Multiple-Output (MIMO) systems. These systems can beset-up having one transmission unit and one reception unit, both unitscomprising more than one transmission or reception antenna,respectively.

Referring to FIG. 1, there is shown a flow diagram of a method fortransmitting a data signal, in particular an orthogonal frequencydivision multiplex (OFDM) signal according to an embodiment. The methodis performed by and within a transmission unit of a wirelessmultiple-input/multiple-output (MIMO) communication system, thecommunication system comprising the transmission unit and a receptionunit, the transmission unit comprising a plurality of transmissionantennas and the reception unit comprising a plurality of receptionantennas. The method comprises performing a first transmission of a datasignal, the first transmission comprising transmitting the data signalby each one of the plurality of the transmission antennas at s1, andperforming a second transmission of the data signal at a time later thanthe first transmission, the second transmission comprising transmittingat least one spectrally modified signal variant of the data signal by atleast one antenna of the plurality of transmission antennas at s2.

The data signal may be an electrical or optical signal carrying amessage or an information. It may be a user signal, or a payload signaltransporting the information in the form of bits. The information bitsmay be modulated onto the data signal by a modulator. The modulator maybe single-carrier modulator, e.g. a QAM modulator or a PSK modulator.The modulator may be a multi-carrier modulator, e.g. an OFDM modulator,a wavelet modulator or a DMT modulator.

An OFDM modulator according to an embodiment receives high rateinformation which is split onto N rate sub-carriers. The data istherefore transmitted by blocks of size N: x(n)=[x₁(n), x₂(n), x_(k)(n),. . . , x_(N)(n)], where the index n is the block OFDM symbol number andthe subscript k is for the carrier index. The block OFDM symbols arecoded by an inverse FFT (Fast Fourier Transformation) matrix to yieldthe so-called time domain block vector s(n)=[s₁(n), s₂(n), s_(k)(n), . .. , s_(N)(n)]. At the output of the inverse FFT, a guard interval(cyclic prefix) of D samples is inserted at the beginning of each block[s_(N−D+1)(n), . . . , s_(N)(n), s_(k)(n), s_(i)(n), . . . , s_(N)(n)].It may consist of a cyclic extension of the time domain OFDM symbol ofsize larger than the channel impulse response (D>L−1). The cyclic prefixis appended between each block in order to transform the multi-pathlinear convolution into a circular one. After Parallel to Serialconversion the data signal, in particular the OFDM signal, is providedby the OFDM modulator in a discrete-time representation. A pre-codingaccording to an embodiment may be applied to the data signal in order tomodify its spectral characteristics. The pre-coded data signal may beconverted from digital to analog and sent through the communicationschannel to the receiver.

At the receiver, symmetrical operations are performed, i.e for an OFDMreceiver down conversion and Analog to Digital Conversion to obtain thediscrete time received signal r^(CP)(n)=[r_(N−D+1)(n), . . . , r_(N)(n),r₁(n), r_(k)(n), . . . , r_(N)(n)]. In order to suppress inter-blockinterference, the first D samples (cyclic prefix or guard intervalsuppression) of the received signal r^(CP)(n) are discarded and theresulting received signal r(n)=[r₁(n), r_(k)(n), . . . , r_(N)(n)] isprocessed by Fast Fourier Transformation to yield the transmitted datasignal y(n) which corresponds to the original data signal x(n)multiplied by the channel matrix H(n) plus a noise term.

According to an embodiment the data signal is an OFDM signal. Accordingto an embodiment the data signal comprises a plurality of signalcomponents, each signal component being spectrally modified to obtain arespective spectrally modified signal variant of the data signal.According to an embodiment the spectrally modified signal variant of thedata signal is obtained by applying a convolutional coding to arespective signal component of the data signal, the convolutional codingbeing represented by a convolutional sequence. The convolutional codingwith a convolutional sequence represents a pre-coding with a pre-codingsequence.

Referring to FIG. 2, there is shown a flow diagram of a method fortransmitting a data signal, in particular an orthogonal frequencydivision multiplex (OFDM) signal according to an embodiment. The methodis performed by and within a transmission unit of a wirelessmultiple-input/multiple-output (MIMO) communication system, thecommunication system comprising the transmission unit and a receptionunit, the transmission unit comprising a plurality of transmissionantennas and the reception unit comprising a plurality of receptionantennas. The method comprises performing a first transmission of thedata signal comprising a plurality of signal components by the pluralityof transmission antennas, each transmission antenna transmitting arespective signal component of the data signal at S1. The method furthercomprises selecting a subset of the plurality of transmission antennasto obtain selected transmission antennas which are transmission antennasbelonging to the subset of the plurality of transmission antennas and toobtain non-selected transmission antennas which are transmissionantennas not belonging to the subset of the plurality of transmissionantennas at S2. The method also includes pre-coding each of the signalcomponents of the data signal transmitted by the selected transmissionantennas with a respective pre-coding sequence to obtain spectrallymodified variants of the signal components of the data signal at S3 andperforming a second transmission of the data signal at a time later thanthe first transmission, the second transmission comprising transmittingby the selected transmission antennas the spectrally modified variantsof the signal components of the data signal and re-transmitting by thenon-selected transmission antennas the signal components of the datasignal transmitted by the non-selected transmission antennas at S4.

According to another embodiment the spectrally modified signal variantsof the signal components of the data signal are transmitted by each ofthe transmission antennas. Thus, no selecting of transmission antennasis required and the method reduces to the two steps of performing afirst transmission of signal components of the data signal at S1 andperforming a second transmission of spectrally modified signal variantsof the signal components of the data signal at a time later than thefirst transmission at S2.

The spectrally modified variant of the signal component of the datasignal to be transmitted from one transmission antenna of the pluralityof transmission antennas is obtained by pre-coding the respective signalcomponent of the data signal with a pre-coding sequence according to anembodiment. The pre-coding sequence may be a convolution sequence. Theconvolution sequence may be applied to the respective signal componentby a mathematical convolution operation which is a time-domainoperation. Alternatively, the pre-coding may be performed in thefrequency domain by applying a frequency-dependent multiplication to therespective signal component of the data signal which isfrequency-transformed. A frequency transformation of the respectivesignal components and/or the pre-coding sequences may be performed byusing a Fourier transformation, e.g. a Fast Fourier Transformation(FFT). The convolution operation may be realized as a delay when theconvolution sequence has a single value at a time unequal to zero. Thedata signal comprises a plurality of signal components, each signalcomponent may be provided at a different one of the transmissionantennas such that the data signal is transmitted by transmission of itssignal components by each of the transmission antennas.

In this application the second transmission will also be called aretransmission. If it happens that the method comprises a secondtransmission and a third transmission, the second transmission will alsobe called the first retransmission and the third transmission will alsobe called the second retransmission.

In order to enhance the reliability of a MIMO link, the technique ofspatial division multiplexing (SDM) is a technique most often associatedwith MIMO. Rather than increasing range, SDM sends different spatialstreams of data from each transmission antenna. Since these streamscarry different data, the overall data rate of the system is increased.The present invention mainly aims to increase the performance of the SDMapproach. SDM schemes require well-conditioned full-rank channelmatrices. In particular for high-order antenna systems, this conditionis often not fulfilled in conventional approaches. The inventionaccording to the embodiment as shown in FIGS. 1 and 2 proposes asolution for the general problem of applying SDM to higher order MIMOsystems.

According to FIGS. 1 and 2, the first transmission is performed withoutany pre-coding. An essential idea of the method includes introducing“artificial” time domain convolutions, namely the respective pre-coding(i.e. convolution) sequences to be transmitted in the secondtransmission of the signal. Such a time domain convolution sequence isexpected to increase the frequency diversity in the second transmissionand will thus help to ensure a more reliable communication.

According to an embodiment of the method of FIGS. 1 or 2, the firsttransmission is represented by a first channel matrix and the secondtransmission is represented by a second channel matrix, and the methodfurther comprises selecting the pre-coding sequences, such that afurther channel matrix can be constructed on the basis of the secondchannel matrix, the further channel matrix comprising a rank which ishigher than the rank of the first channel matrix. According to anembodiment the first channel matrix is rank-deficient. The rank of an mx n matrix is at most min(n,m). A matrix that has a rank as large aspossible is said to have full rank; otherwise, the matrix isrank-deficient. According to a further embodiment the first channelmatrix is quasi rank-deficient, i.e. at least one eigenvalue of thefirst channel matrix is close or very close to zero but not exactlyzero. According to the embodiments above the first channel matrix ispoorly conditioned. A poorly conditioned matrix is a matrix which isquasi rank-deficient or even rank-deficient. In the case of quasirank-deficient first channel matrices the same problems arise as forrank-deficient matrices which problems may be solved by embodiments ofthe invention as described in the instant application.

According to an embodiment of the method of FIGS. 1 or 2, during thesecond transmission each one of the plurality of transmission antennastransmits a spectrally modified variant of a signal component of thedata signal. According to an embodiment thereof, the spectrally modifiedvariants of different antennas are different from each other. Accordingto an embodiment some of the convolution sequences may have only onesingle element such that a respective spectrally modified variant of asignal component of the data signal corresponds to the signal componentof the data signal or to an amplified version thereof.

According to an embodiment of the method of FIGS. 1 or 2, spectrallymodified signal variants may be transmitted by one antenna of theplurality of antennas, all antennas of the plurality of antennas, or anynumber of antennas between one and all of the plurality of antennas.According to an embodiment at least two of the transmission antennashave a different polarization, e.g. vertical or horizontal polarization.

According to an embodiment of the method of FIGS. 1 or 2, the length(duration) of the pre-coding sequences is shorter than an OFDM guardinterval. For example, the length may be 1, 2 or 3 samples. According toan embodiment the length of the pre-coding sequences is shorter than alength of a channel impulse response of the MIMO communication system,in particular shorter than a length of a channel impulse responsebetween the plurality of transmission antennas and the plurality ofreception antennas.

According to an embodiment of the method of FIGS. 1 or 2, the secondtransmission is initiated upon receipt of an information that thetransmitted OFDM signal could not be correctly decoded in a receptionunit on the basis of the first transmission. According to an embodimenta hybrid automatic repeat request procedure is used for initiatingre-transmission.

According to an embodiment of the methods of FIGS. 1 or 2, the methodfurther comprises performing a third transmission of the data signal, inparticular the OFDM signal at a time later than the second transmission,the third transmission comprising transmitting spectrally modifiedsignal variants of the data signal. According to an embodiment thereof,the third transmission is initiated upon receipt of an information thatthe transmitted data signal could not be correctly decoded in areception unit on the basis of the first and second transmissions.According to a further embodiment thereof, the third transmission issimilar or equal to the second transmission. According to an alternativeembodiment, the third transmission is different to the secondtransmission, wherein in particular the spectrally modified signalvariants of the third transmission are obtained by frequency shiftingthe spectrally modified signal variants of the second transmission. Inparticular the spectrally modified signal variants of the thirdtransmission can be obtained by applying a linear phase in the timedomain to each sample value of the spectrally modified signal variantsof the second transmission by multiplying each sample value by e^(iαn),where α is a complex constant and n is the sample number, wherein α canbe equal to π.

According to an embodiment of the method of FIG. 2 the spectrallymodified signal variants of signal components of the data signal of thethird transmission are obtained by selecting a different subset oftransmission antennas for transmission of the spectrally modified signalvariants for the third transmission with respect to the subset selectedfor the second transmission.

According to an embodiment of the method of FIGS. 1 or 2, thetransmission unit comprises a plurality of stored pre-coders, whereineach pre-coder comprises information on spectrally modified signalvariants of a second or third transmission, the method furthercomprising accidentally selecting one of the plurality of storedpre-coders for performing the second or third transmission.

According to an embodiment of the method of FIGS. 1 or 2, thetransmission unit comprises a plurality of stored pre-coders, whereineach pre-coder comprises information on spectrally modified signalvariants of a second or third transmission, the method furthercomprising specifically selecting one particular pre-coding(convolutional) sequence out of the plurality of stored pre-coding(convolutional) sequences for performing the second or thirdtransmission.

Referring to FIG. 3, there is shown a flow diagram of a method forreceiving a data signal, in particular an orthogonal frequency divisionmultiplex (OFDM) signal by a reception unit of a wirelessmultiple-input/multiple-output (MIMO) communication system, thecommunication system comprising the reception unit and a transmissionunit, the transmission unit comprising a plurality of transmissionantennas and the reception unit comprising a plurality of receptionantennas. The method comprises receiving a first transmission of a datasignal, the first transmission being represented by a first channelmatrix at s1, and receiving a second transmission of the data signal ata time later than the first transmission, the second transmissioncomprising spectrally modified signal variants of signal components ofthe data signal and being represented by a second channel matrix at s2.The method of FIG. 3 further compriese constructing a further channelmatrix on the basis of the second channel matrix, the further channelmatrix comprising a rank which is higher than a rank of the firstchannel matrix at s3, and decoding the transmitted data signal by meansof the further channel matrix at s4.

According to an embodiment of the method of FIG. 3, the method furthercomprises requesting the second transmission after detecting that thetransmitted data signal can not be correctly decoded on the basis of thefirst transmission. The detecting, for example, can be done by standarderror-detection processes in which error-detection information bits areadded to data to be transmitted. A standard procedure is the well-knowncyclic redundancy check (CRC). In the case of detection of an error, arequest for a repeated transmission is automatically generated, theprocedure is thus called automatic repeat request (ARQ). In a furthervariant thereof, which is called hybrid ARQ (HARQ), forward errorcorrection (FEC) bits are also added to the existing error detectionbits.

According to an embodiment of the method of FIG. 3, the method furthercomprises constructing the further channel matrix on the basis of thefirst channel matrix and the second channel matrix.

Referring to FIG. 4, there is shown a schematic block representation ofa wireless MIMO system performing an embodiment of the method depictedin FIGS. 1 or 2. A data signal s(n) comprises a plurality of signalcomponents s₁(n), s₂(n), s₃(n) and s₄(n). During a first transmissioneach of the signal components s₁(n), s₂(n), s₃(n) and s₄(n) istransmitted by a respective transmission antenna TA₁, TA₂, TA₃ and TA₄ofa transmission unit TU of a wireless MIMO communication system. For thesecond transmission a subset (SUBSET) of the plurality of transmissionantennas TA₁, TA₂, TA₃ and TA₄ is selected to obtain selectedtransmission antennas (TA₁, TA₄) which belong to the subset and toobtain non-selected antennas (TA₂, TA₃) which do not belong to thesubset. Each of the signal components s₁(n) and s₄(n) of the data signals(n) transmitted by the selected transmission antennas (TA₁, TA₄) arepre-coded with a respective pre-coding sequence (g₁(n), g₄(n)) to obtainspectrally modified variants (s^₁(n), s{circumflex over (0)}₄(n)) of thesignal components of the data signal s(n). The second transmission isperformed at a time later than the first transmission and comprisestransmitting by the selected transmission antennas (TA₁, TA₄) thespectrally modified variants (s{circumflex over (0 )}₁(n), s{circumflexover (0)}₄(n)) of the signal components of the data signal s(n) andre-transmitting by the non-selected transmission antennas (TA₂, TA₃) thesignal components (s₂(n), s₃(n)) of the data signal s(n) transmitted bythe non-selected transmission antennas (TA₂, TA₃) during the firsttransmission.

Referring to FIG. 5, there is shown a schematic block representation ofa wireless MIMO system performing an embodiment of the methods depictedin FIGS. 1, 2 and 3. A data signal s(k) represented in frequency domainby the index k comprises a plurality of signal components s₁(k), s₂(k),s₃(k) and s₄(k). During a first transmission which is designated by thesuperscript notation (1) each of the signal components s₁(k), s₂(k),s₃(k) and s₄(k) is transmitted by a respective transmission antenna TA₁,TA₂, TA₃ and TA₄ of a transmission unit TU of a wireless MIMOcommunication system. A reception unit RU of the MIMO communicationsystem comprising a plurality of reception antennas RA₁, RA₂ receivesthe transmitted signal components s₁(k), s₂(k), s₃(k) and s₄(k) as aplurality of received signal components r₁ ⁽¹⁾(k), r₂ ⁽¹⁾(k) of areceived signal r⁽¹⁾(k). Each received signal component r_(i) ⁽¹⁾(k)depends on the transmitted signal component s_(j)(k) by the respectivechannel path (channel impulse response) H_(ji). The relation can bewritten as r_(i) ⁽¹⁾(k)=H_(ij) s_(j)(k). The index j specifies thetransmission antenna TX_(j) and the index i specifies the receptionantenna RX; of the respective transmission path. The plurality ofindividual channel impulse responses H_(ji) may be represented as asingle matrix H⁽¹⁾ which is designated as the first channel matrix.

During a second transmission (2) at a time later than the firsttransmission (1) spectrally modified variants s₁{circumflex over(0)}(k), s₂{circumflex over (0)}(k), s₃{circumflex over (0)}(k),s₄{circumflex over (0)}(k) of the signal components s₁(k), s₂(k), s₃(k),s₄(k) of the data signal s(k) are transmitted by a respectivetransmission antenna TA₁, TA₂, TA₃ and TA₄ of the transmission unit TUof the wireless MIMO communication system. The reception unit RU of theMIMO communication system receives the transmitted spectrally modifiedvariants of the signal components s₁(k), s₂(k), s₃(k) and s₄(k) as aplurality of received signal components r₁ ⁽²⁾(k), r₂ ⁽²⁾(k) of areceived signal r⁽²⁾(k). Each received signal component r₁ ⁽²⁾(k)depends on the respective spectrally modified variant s_(j){circumflexover (0 )}(k) by the respective channel path (channel impulse response)H_(ji). Each spectrally modified variant s_(j){circumflex over (0)}(k)depends on the respective signal component s_(j)(k) of the data signals(k) by the respective pre-coder G_(j) which is the frequencyrepresentation of the pre-coding sequence g_(j)(n). The relation can bewritten as s_(j){circumflex over (0)}(k)=G_(j)s_(j)(k) and as r_(i)⁽²⁾(k)=H_(ji) s_(j){circumflex over (0)}(k). The plurality of individualchannel impulse responses H_(ji) and the plurality of individualpre-coders G_(j) may be represented as a single matrix H⁽²⁾ which isdesignated as the second channel matrix. According to one embodimentdescribed not each of the transmission antennas TX_(j) may transmit aspectrally modified variant s_(j){circumflex over (0)}(k) of a signalcomponent s_(j)(k) of the data signal s(k), some of the transmissionantennas TX_(j) may transmit an original signal component s_(j)(k) ofthe data signal s(k).

A further channel matrix H^((*)) may be constructed on the basis of thesecond channel matrix H⁽²⁾ by a combination of the first channel matrixH⁽¹⁾ and the second channel matrix H⁽²⁾. FIG. 5 depicts one possible wayof combining both matrices. The first received signal component r₁⁽¹⁾(k) of the first transmission which is received by the first antennaRA₁ is added to the first received signal component r₁ ⁽²⁾(k) of thesecond transmission which is received by the first antenna RA₁ toreceive a first received signal at 100. A second received signal isconstructed at 102 as the second received signal component r₂ ⁽¹⁾(k) ofthe first transmission which is received by the second antenna RA₂. Thiscombination results in a further channel matrix H^((*)) having a rankwhich may be controlled by adequate adjusting the pre-coders G_(j). Forexample the adjustment may be used for obtaining a full rank or amaximum rank further channel matrix H^((*)).

According to an embodiment as depicted in FIG. 3 or 5, a method forconstructing a well-conditioned further channel matrix from a poorlyconditioned first channel matrix and parts of a second poorlyconditioned channel matrix comprises constructing a first channel matrixon the basis of received samples of a data signal comprising a pluralityof signal components transmitted by a plurality of transmission antennasand received by a plurality of reception antennas, the first channelmatrix being poorly conditioned. The method further comprisesconstructing a second channel matrix on the basis of received samples ofspectrally modified variants of signal components of the data signalbeing transmitted by the plurality of transmission antennas and receivedby the plurality of reception antennas, the second channel matrix beingpoorly conditioned. The method further comprises constructing a furtherchannel matrix on the basis of the received samples of the signalcomponents of the data signal and on the basis of parts of the receivedsamples of the spectrally modified variants of the signal components ofthe data signal such that the further channel matrix iswell-conditioned.

According to an embodiment, at least one of the first channel matrix andthe second channel matrix is poorly conditioned. A poorly conditionedmatrix is a matrix which is quasi rank-deficient, i.e. at least oneeigenvalue of the matrix is close or very close to zero. Arank-deficient matrix is also considered a poorly conditioned matrix.The rank of an m×n matrix is at most min(n,m). A matrix that has a rankas large as possible is said to have full rank; otherwise, the matrix isrank-deficient. A well-conditioned matrix is a matrix which is neitherrank-deficient nor quasi rank-deficient. Well-conditioned matrices maybe used for matrix inversion.

In the following a specific embodiment similar to the embodimentdepicted in FIG. 5 will be described in further detail. The pre-coding(convolution) sequences which are defined in the time domain areproposed to be chosen with respect to the following criteria:

The convolution sequence to be applied to TX antenna #j is typicallydefined by a vector containing complex elements:g ^((j))=(g ₀ ^((j)) g ₁ ^((j)) . . . ), g _(n) ^((j)) εC∀n.

The sequence g^((j)) is typically short, i.e. only elements g_(n) ^((j))with small “n” are non-zero (typically g_(n>M) ^((j))=0 with M<<N,wherein M is two orders of magnitude smaller than N, with N being thesize of the OFDM symbol, e.g. N=2048 in one 3GPP LTE configuration). Amore specific “shortness” condition is given below.

As a result, the receiver perceives the additional convolution as partof the overall system impulse response including the over-the-airchannel impulse response convolved with the RF transmitter and receiverfilter impulse responses convolved with the corresponding sequenceg^((j)). g^((j)) is chosen to be short in the sense that the resultingsystem impulse response is still smaller compared to the OFDM guardinterval (or cyclic prefix), thus no additional steps are required fortaking it into account for the decoding in the receiver unit. Thereceiver unit does not even have to be “aware” of g^((k)) since itconsiders it to be part of the overall system impulse response whichneeds to be equalized anyhow. In 3GPP LTE, the guard interval size forthe DL communication is typically 140 or 166 samples and an extendedguard interval of 512 or 1024 samples can also be used, i.e. atransmitter unit convolutional pre-coding is typically lengthening theoverall system impulse response irrelevantly.

Referring to FIG. 6, there is shown a diagram to illustrate theintroduction of frequency diversity. As stated above, the objective ofthe introduction of a time domain convolution sequence is to introducethe overall frequency diversity. In FIG. 6 the frequency response isillustrated for the case of just one (strong) Line-of-Sight (LOS)component and the frequency response resulting in the combination ofsuch a channel with a simple transmitter convolution, wherein thefollowing convolution sequence is used: g⁽¹⁾=(1 2)/√{square root over(5)}).

Referring to FIG. 7, there is shown a diagram to illustrate theintroduction of frequency diversity, including a frequency shiftedversion of the original convolutional sequence used for the firstretransmission. If the message can not be decoded after the firstretransmission, another retransmission is required. In this case, it isproposed that the same convolution sequences are applied as for thefirst retransmission with the difference that a frequency shift of thissequence is performed (in order to avoid that bits fall into “fadings”for a second time). Such a frequency domain shift is performed byapplying a linear phase in the time domain, i.e. each sample number “n”is multiplied by e^(jαn) with α being a complex constant. A preferredsolution is to choose α=π, leading to a cyclic spectral shift by half ofthe bandwidth.

As already indicated above, according to an embodiment it is proposedthat the convolution sequences g^((k)) are specifically selected suchthat the respective condition of the channel matrices are improved whenthe received symbols of the first transmission and the received symbolsof the second transmission are combined.

The MIMO communication system may be described in a frequency domainrepresentation by assuming thatr=Hs+zwherein s is the N_(TX)×1 signal vector (in frequency domainrepresentation) transmitted by the transmission unit which correspondsto the data signal (in frequency domain representation), r is theN_(RX)×1 signal vector (in frequency domain representation) received bythe reception unit, H is the N_(RX)×N_(TX) channel matrix, and z is theN_(RX)×1 noise vector (in frequency domain representation). The N_(TX)×1signal vector s is denominated as the data signal and the N_(TX)elements of s are denominated as the signal components of the datasignal. The data signal as well as the signal components of the datasignal may be represented in the time domain or in the frequency domain.Reconstructing the transmitted signal vector s by the received signalvector r and by this demodulation of s may be performed by inverting thechannel matrix H. Such a matrix inversion requires that H has no rankdeficiencies.

We further assume that H is rank-deficient and s can not be demodulatedeven if s is retransmitted. It is then proposed to use an independentpre-coder/processor matrix P for the retransmission. In one exemplaryembodiment the MIMO communication system is a 2×2 MIMO system whereinthe transmission unit comprises two transmission antennas and thereception unit comprises two reception antennas. Of course, allmechanisms described hereinafter can be straightforwardly extended toany dimension.

Transmit #1 with a pre-coder P₁, so thatr ₁ =HP ₁ s+z ₁.

Transmit #2 with pre-coder P₂, so thatr ₂ =HP ₂ s+z ₂.

We can thus write

$\begin{bmatrix}r_{1} \\r_{2}\end{bmatrix} = {{\begin{bmatrix}{HP}_{1} \\{HP}_{2}\end{bmatrix}\mspace{14mu} s} + \begin{bmatrix}z_{1} \\z_{2}\end{bmatrix}}$

Then it is possible to choose P₁, P₂ such that all elements of s can bedemodulated, namely if

${{{rank}\left\{ \begin{bmatrix}{HP}_{1} \\{HP}_{2}\end{bmatrix} \right\}} \geq {N_{TX}.}}\mspace{11mu}$

In the following example the use of a convolutional decoder isillustrated.

A 2×2 MIMO system is considered, using Spatial Division Multiplexingwith two spatial streams, which means that a full-rank (i.e. rank 2)preferably well-conditioned, channel matrix H_(N) _(RX) _(×N) _(TX)^((k)) is required for each OFDM carrier “k” (H_(N) _(RX) _(×N) _(TX)^((k)) contains all channel attenuation between all transmission andreception antennas).

To illustrate a worst case scenario, it is assumed that the channelsbetween the various transmission and reception antennas arequasi-identical, e.g. due to very strong LOS (Line-of-Sight) componentsand corresponding positioning of the antennas. Thus, in this worst casescenario, the channel matrices would correspond to

$H_{N_{RX} \times N_{TX}}^{(k)} = {\begin{bmatrix}H_{11}^{(k)} & H_{12}^{(k)} \\H_{21}^{(k)} & H_{22}^{(k)}\end{bmatrix} = {\begin{bmatrix}1 & 1 \\1 & 1\end{bmatrix}{\forall{k.}}}}$

Obviously this matrix is rank deficient as only one singular value isnon-zero as it can be verified by calculating theSingular-Value-Decomposition (SVD). Consequently the transmission of twospatial streams will fail for all carriers. In such a context even thetransmission of redundancy information is in vain, since the symbols cannot be decoded as long as the channel impulse response does not change.

In order to circumvent this problem, for each retransmission of theoriginal data, the introduction of a small time domain convolution priorto the transmission by antenna #1 is introduced. For this example, thefollowing normalized convolution sequence is chosen: g⁽¹⁾=(1 2)/√{squareroot over (5)}. Also an OFDM symbol with 2048 carriers is assumed(following the 3GPP LTE standard). We now have two channel matrices, onefor the initial transmission and one for the retransmission:

${{Initial}\mspace{14mu}{transmission}\text{:}\mspace{14mu} H_{N_{RX} \times N_{TX}}^{(k)}} = {\begin{bmatrix}H_{11}^{(k)} & H_{12}^{(k)} \\H_{21}^{(k)} & H_{22}^{(k)}\end{bmatrix} = {\begin{bmatrix}1 & 1 \\1 & 1\end{bmatrix}{\forall k}}}$${1.\mspace{14mu}{Retransmission}\text{:}\mspace{14mu} H_{N_{RX} \times N_{TX}}^{{(k)},{R\; 1}}} = {\begin{bmatrix}{{c(k)} \cdot H_{11}^{(k)}} & H_{12}^{(k)} \\{{c(k)} \cdot H_{21}^{(k)}} & H_{22}^{(k)}\end{bmatrix} = {\begin{bmatrix}{c(k)} & 1 \\{c(k)} & 1\end{bmatrix}{\forall k}}}$where “c(k)” corresponds to the multiplicative element (in frequencydomain) introduced by the convolution sequence g⁽¹⁾=(1 2)/√{square rootover (5)} (in time domain). c(k) further corresponds to the pre-coderG_(j) as described in FIG. 5. The convolution by g⁽¹⁾=(1 2)/√{squareroot over (5)} is performed in time domain represented by the index “n”,leading to a multiplicative factor c(k) in frequency domain representedby the index “k”. The calculations below are performed in frequencydomain for all carriers “k” (as indicated by the channel matrix H_(N)_(RX) _(×N) _(TX) ^((k)). The multiplicative factor “c(k)” may bederived from g⁽¹⁾ by a Fourier Transformation.

It is now proposed that two received symbols are constructed based onthese channel matrices, one by combining both initially received symbolsplus the received symbol of the first reception antenna and another oneby combining both initially received symbols plus the received symbol ofthe second reception antenna. This leads to new channel matricescorresponding to the two new constructed symbols:

${\left. {{{\left. 1 \right)\mspace{14mu}{new}\mspace{14mu}{constucted}\mspace{14mu}{matrix}\text{:}\mspace{14mu} H_{N_{RX} \times N_{TX}}^{{(k)},{N\; 1\alpha}}} = {\left\lbrack \begin{matrix}{\left( {1 + {c(k)}} \right) \cdot H_{11}^{(k)}} & {2H_{12}^{(k)}} \\H_{21}^{(k)} & H_{22}^{(k)}\end{matrix} \right\rbrack = {\begin{bmatrix}{1 + {c(k)}} & 2 \\1 & 1\end{bmatrix}{\forall k}}}}2} \right)\mspace{14mu}{new}\mspace{14mu}{constructed}\mspace{14mu}{matrix}\text{:}\mspace{14mu} H_{N_{RX} \times N_{TX}}^{{(k)},{N\; 1b}}} = {\left\lbrack \begin{matrix}H_{11}^{(k)} & H_{12}^{(k)} \\{\left( {1 + {c(k)}} \right) \cdot H_{21}^{(k)}} & {2H_{22}^{(k)}}\end{matrix} \right\rbrack = {\left\lbrack \begin{matrix}1 & 1 \\{1 + {c(k)}} & 2\end{matrix} \right\rbrack{\forall k}}}$

The second equality of 1) and 2) is only valid if the condition

$H_{N_{RX} \times N_{TX}}^{(k)} = {\begin{bmatrix}H_{11}^{(k)} & H_{12}^{(k)} \\H_{21}^{(k)} & H_{22}^{(k)}\end{bmatrix} = {\begin{bmatrix}1 & 1 \\1 & 1\end{bmatrix}{\forall k}}}$is met as described above.

The proposed scheme thus allows to ensure a robust communication for SDMeven in the presence of highly correlated MIMO channels. In other words,it avoids the occurrence of ill-conditioned channel matrices. If furtherretransmissions are required, the transmitter can “randomly” adjust theconvolution sequences without informing the receiver about its nature.Simply by constructing various new decoding matrices, the receiver willbe able to extract the information. Those further retransmissions mightbe put into spatial streams/beams with a low SNR. Only a littleadditional energy (incremental redundancy) might be required to fullydecode the packet. This can be done by properly choosing a convolutionalpre-coder, i.e. the convolution sequence.

The upper formulation is quite general. In fact, the example

${{{initial}\mspace{14mu}{transmission}\text{:}\mspace{14mu} H_{N_{RX} \times N_{TX}}^{(k)}} = {\begin{bmatrix}1 & 1 \\1 & 1\end{bmatrix}{\forall k}}},{and}$${{retransmission}\text{:}\mspace{14mu} H_{N_{RX} \times N_{TX}}^{{(k)},{R\; 1}}} = {\begin{bmatrix}{c(k)} & 1 \\{c(k)} & 1\end{bmatrix}{\forall k}}$can be considered as a special case of this formulation, namely

$\mspace{79mu}{{{Transmit}\mspace{14mu}{\# 1}\text{:}\mspace{14mu} P_{1}} = {\left. \begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}\Rightarrow{{equivalent}\mspace{14mu}{channel}\mspace{14mu}{HP}_{1}} \right. = \begin{bmatrix}1 & 1 \\1 & 1\end{bmatrix}}}$${{Transmit}\mspace{14mu}{\# 2}\text{:}\mspace{14mu} P_{2}} = {\left. \begin{bmatrix}{c(k)} & 0 \\0 & 1\end{bmatrix}\Rightarrow{{equivalent}\mspace{14mu}{channel}\mspace{14mu}{HP}_{2}} \right. = \begin{bmatrix}{c(k)} & 1 \\{c(k)} & 1\end{bmatrix}}$when c(k)≠1, then

${{{rank}\left\{ \begin{bmatrix}{HP}_{1} \\{HP}_{2}\end{bmatrix} \right\}} = 2},$so that s can be demodulated.

Devices according to embodiments improve the system performance byincrease of the channel diversity without CSI (Channel StateInformation) knowledge in the transmission unit. By using a pre-coding(convolutional coding) in the transmission unit the overall channeldiversity is efficiently increased. The introduction of this techniquedoes not need to be communicated to the reception unit, nor does it needto be known by the reception unit, since it is inherently taken intoaccount by the channel decoding procedures. However, according to anembodiment the pre-coder may be designed by using CSI. By exploitingknowledge about CSI the system performance may be further improved.

Embodiments of MIMO systems as illustrated in the figures provide meanson how to transform poorly conditioned and rank deficient channelmatrices into well conditioned ones enabling an efficient decoding ofthe transmitted information. Embodiments of such MIMO systems may becombined with common MIMO techniques and HARQ processing.

For special channel

${H_{N_{RX} \times N_{TX}}^{(k)} = {\begin{bmatrix}0 & 1 \\0 & 1\end{bmatrix}{\forall k}}},$then

${HP}_{2} = \begin{bmatrix}0 & 1 \\0 & 1\end{bmatrix}$with

$P_{2} = \left. \begin{bmatrix}{c(k)} & 0 \\0 & 1\end{bmatrix}\Rightarrow \right.$this P2 does not work since

${{rank}\left\{ \begin{bmatrix}{HP}_{1} \\{HP}_{2}\end{bmatrix} \right\}} = 1.$However, we can choose

${P_{2} = \begin{bmatrix}{c(k)} & 0 \\0 & 1\end{bmatrix}},$then

${HP}_{2} = {\begin{bmatrix}{c(k)} & 1 \\{c(k)} & 1\end{bmatrix}.}$When c(k)≠1, then

${{{rank}\left\{ \begin{bmatrix}{HP}_{1} \\{HP}_{2}\end{bmatrix} \right\}} = 2},$thus s can be demodulated.

It is to be noted that the upper way of constructing the matrices

$H_{N_{RX} \times N_{TX}}^{{(k)},{N\; 1\alpha}} = {\left\lbrack \begin{matrix}{\left( {1 + {c(k)}} \right) \cdot H_{11}^{(k)}} & {2H_{12}^{(k)}} \\H_{21}^{(k)} & H_{22}^{(k)}\end{matrix} \right\rbrack = {\begin{bmatrix}{1 + {c(k)}} & 2 \\1 & 1\end{bmatrix}{\forall k}}}$$H_{N_{RX} \times N_{TX}}^{{(k)},{N\; 1b}} = {\left\lbrack \begin{matrix}H_{11}^{(k)} & H_{12}^{(k)} \\{\left( {1 + {c(k)}} \right) \cdot H_{21}^{(k)}} & {2H_{22}^{(k)}}\end{matrix} \right\rbrack = {\begin{bmatrix}1 & 1 \\{1 + {c(k)}} & 2\end{bmatrix}{\forall k}}}$requires a “suitable” c(k) selection, i.e. a “suitable” selection of thepre-coder. In particular, the rank properties are not improved if thefollowing condition holds: (1+c(k))=2. This is in particular true, ifthe pre-coder is entirely omitted. But even in presence of a pre-coder,this condition should be omitted for all carriers.

The proof for the poor matrix rank improvement results is as follows:With (1+c(k))=2, the resulting matrices can be written as follows:

$H_{N_{RX} \times N_{TX}}^{{(k)},{N\; 1\alpha}} = {\left\lbrack \begin{matrix}{\left( {1 + {c(k)}} \right) \cdot H_{11}^{(k)}} & {2H_{12}^{(k)}} \\H_{21}^{(k)} & H_{22}^{(k)}\end{matrix} \right\rbrack = {\left\lbrack \begin{matrix}{2 \cdot H_{11}^{(k)}} & {2H_{12}^{(k)}} \\H_{21}^{(k)} & H_{22}^{(k)}\end{matrix} \right\rbrack = {\quad{{{\left\lbrack \begin{matrix}2 & 0 \\0 & 1\end{matrix} \right\rbrack\left\lbrack \begin{matrix}H_{11}^{(k)} & {2H_{12}^{(k)}} \\H_{21}^{(k)} & H_{22}^{(k)}\end{matrix} \right\rbrack}{\forall{k\mspace{20mu}{and}H_{N_{RX} \times N_{TX}}^{{(k)},{N\; 1b}}}}} = {\left\lbrack \begin{matrix}H_{11}^{(k)} & H_{12}^{(k)} \\{\left( {1 + {c(k)}} \right) \cdot H_{21}^{(k)}} & {2H_{22}^{(k)}}\end{matrix} \right\rbrack = {\left\lbrack \begin{matrix}H_{11}^{(k)} & H_{12}^{(k)} \\{2H_{21}^{(k)}} & {2H_{22}^{(k)}}\end{matrix} \right\rbrack = {\quad{{\left\lbrack \begin{matrix}1 & 0 \\0 & 2\end{matrix} \right\rbrack\left\lbrack \begin{matrix}H_{11}^{(k)} & {2H_{12}^{(k)}} \\H_{21}^{(k)} & H_{22}^{(k)}\end{matrix} \right\rbrack}{\forall{k.}}}}}}}}}}$

Due to the well-known matrix calculus rule “rank(AB) rank(A)” and“rank(AB)≦rank(B)” the upper steps cannot increase the rank of theoriginal rank-deficient transmission

${H_{N_{RX} \times N_{TX}}^{(k)} = {\left\lbrack \begin{matrix}H_{11}^{(k)} & H_{12}^{(k)} \\H_{21}^{(k)} & H_{22}^{(k)}\end{matrix} \right\rbrack = {\begin{bmatrix}1 & 1 \\1 & 1\end{bmatrix}{\forall k}}}},{since}$ ${{rank}\left( {\begin{bmatrix}2 & 0 \\0 & 1\end{bmatrix}\left\lbrack \begin{matrix}H_{11}^{(k)} & H_{12}^{(k)} \\H_{21}^{(k)} & H_{22}^{(k)}\end{matrix} \right\rbrack} \right)} \leq {{{rank}\left( \left\lbrack \begin{matrix}H_{11}^{(k)} & H_{12}^{(k)} \\H_{21}^{(k)} & H_{22}^{(k)}\end{matrix} \right\rbrack \right)}\mspace{14mu}{and}}$${{rank}\left( {\begin{bmatrix}1 & 0 \\0 & 2\end{bmatrix}\left\lbrack \begin{matrix}H_{11}^{(k)} & H_{12}^{(k)} \\H_{21}^{(k)} & H_{22}^{(k)}\end{matrix} \right\rbrack} \right)} \leq {{{rank}\left( \left\lbrack \begin{matrix}H_{11}^{(k)} & H_{12}^{(k)} \\H_{21}^{(k)} & H_{22}^{(k)}\end{matrix} \right\rbrack \right)}.}$

While the optimum solution requires the knowledge of the channel andthus an information exchange prior to the usage of the method, asimplified version can be implemented by the vendor for HARQchase-combining: With every retransmission any predeterminedconvolutional pre-coder may be employed without notification of thereceiver. If the receiver is aware of the proposed mechanisms, it canimprove the link characteristics following our proposals in thisapplication. If the receiver is unaware (e.g. competitor's product), thestandard decoding approaches will work since the pre-coder is “hidden”in the overall system impulse response.

In general, the following should be considered for selection of theconvolutional pre-coders in the optimum case, i.e. channelcharacteristics are known at transmitter.

Pre-coder P₂ at retransmission should be chosen (e.g. orthogonal to P₁)such that the resulting compound pre-coder-channel matrix, possiblyindependent of the real channel, has the highest/higher rank.

In order to achieve a), the convolutional pre-coder needs to be designedjointly for all transmission antennas, not separately for individualantennas.

The convolutional pre-coder is a linear pre-coder. It should thereforebe possible to define a good convolutional pre-coder, depending on thechannel, as well as the receiver.

Referring to FIG. 8, there is shown a schematic block presentation of aMIMO system in a generalized form. The MIMO system comprises atransmission unit 10 having a plurality of transmission antennas 11 to14, and a reception unit 20 having a plurality of reception antennas and22. There are also shown some but not all of the spatial beams from thetransmission antennas 11 to 14 to the reception antennas 21 and 22.

Referring to FIG. 9, there is shown a schematic block presentation of atransmission unit to transmit a data signal, in particular an orthogonalfrequency division multiplex (OFDM) signal. The transmission unit 10comprises a plurality of antennas 11, 12, 13, and 14, a firsttransmission device (TX1) 15 to perform a first transmission of a datasignal, the first transmission comprising transmitting the data signalby the plurality of transmission antennas 11 to 14, and a secondtransmission device (TX2) 16 to perform a second transmission of thedata signal at a time later than the first transmission, the secondtransmission comprising transmitting signal variants of the data signalby at least one antenna of the plurality of transmission antennas 11 to14, wherein the second transmission device 16 comprises a pre-codingdevice (PCD) 16.1 to pre-code the data signal with a respectiveconvolutional sequence to obtain a respective signal variant.

According to an embodiment of the transmission unit 10 of FIG. 9, thetransmission unit 10 further comprises a third transmission device (TX3)17 to perform a third transmission of the data signal at a time laterthan the second transmission, the third transmission comprisingtransmitting spectrally modified signal variants of the data signal.According to an embodiment thereof, the third transmission device 17 isarranged to initiate the third transmission upon receipt of aninformation that the transmitted data signal could not be correctlydecoded on the basis of the first and second transmissions. According toan embodiment, the third transmission device 17 is arranged to perform athird transmission which is similar or equal to the second transmission.According to an alternative embodiment, the third transmission device 17is arranged to perform a third transmission which is different to thesecond transmission. In particular, the third transmission device 17 isarranged to perform a third transmission, the signal variants thereofare obtained by frequency shifting the signal variants of the secondtransmission. In particular, the signal variants of the thirdtransmission are obtained by applying a linear phase in time domain toeach sample value of the signal variants of the second transmission bymultiplying each sample value by e^(jαn), where α is a complex constantand n is the sample number, wherein α can be equal to π.

According to an embodiment of the transmission unit 10 of FIG. 9, thetransmission 10 further comprises a storage device 18 to store aplurality of pre-coders, wherein each pre-coder comprises information onspectrally modified signal variants of a second or third transmission.

According to an embodiment of the transmission unit 10 of FIG. 9, thetransmission unit 10 further comprises a selection device 19 to select apre-coder for performing a second or third transmission. According to anembodiment thereof, the selection device 19 is arranged to specificallyselect a particular pre-coder on the basis of an information about thechannel in form of the channel matrix. The selection device 19 can alsobe arranged to accidentally select an arbitrary decoder.

Further embodiments of the transmission unit 10 of FIG. 9 can beconstructed in which the devices of the transmission unit 10 arearranged to perform operations as described above in connection with amethod for transmitting a data signal according to FIG. 1 or 2.

According to further embodiments of the transmission unit 10 of FIG. 9,the devices of the transmission unit 10 can be either implemented ashardware elements or as software tools within a processor like a digitalsignal processor (DSP). It is also possible that part of the devices canbe implemented has hardware elements and the other part can beimplemented has software tools.

Referring to FIG. 10, there is shown a schematic block representation ofa reception unit 20 to receive a data signal, in particular anorthogonal frequency division multiplex (OFDM) signal. The receptionunit 20 comprises a plurality of antennas 21 and 22, a reception device(RX) 23 to receive a first transmission of a data signal and a secondtransmission of the data signal, wherein the first transmission isrepresented by a first channel matrix and the second transmissioncomprises spectrally modified signal variants of the data signal and isrepresented by a second channel matrix, a construction device (CSTR) 24to construct a further channel matrix on the basis of the second channelmatrix, the further channel matrix comprising a rank which is higherthan the rank of the first channel matrix, and a decoding device (DEC)25 to decode the transmitted data signal by means of the further channelmatrix.

According to an embodiment of the reception unit 20 of FIG. 10, thereception unit 20 further comprises a detection device (DET) 26 todetect that the transmitted data signal can not be correctly decoded onthe basis of the first transmission, and a request device (REQ) 27 torequest the second transmission, the request device 27 coupled to thedetection device 20. According to an embodiment thereof, the detectiondevice 20 is arranged to operate on the basis of error-detectioninformation bits added to the transmitted data, in particular inconnection with cyclic redundancy check (CRC). According to a furtherembodiment, the detection device 26 is arranged to operate on the basisof both error-detection information bits and forward error correctionbits added to the error-detection bits.

Further embodiments of the reception unit 20 can be constructed in whichthe devices of the reception unit 20 are arranged to perform operationsas were described above in connection with the method for receiving anOFDM signal according to signal 2.

What is claimed is:
 1. A method for transmitting an orthogonal frequencydivision multiplex (OFDM) data signal by a transmission unit of awireless multiple-input/multiple-output (MIMO) communication system, thetransmission unit comprising a plurality of transmission antenna ports,the method comprising: performing a first transmission of an OFDM datasignal, the first transmission comprising transmitting the OFDM datasignal by each one of the plurality of transmission antenna ports;performing a second transmission of the OFDM data signal at a time laterthan the first transmission, the second transmission comprisingtransmitting at least one spectrally modified signal variant of the OFDMdata signal by at least one transmission antenna port of the pluralityof transmission antenna ports, wherein the first transmission isrepresented by a first channel matrix and the second transmission isrepresented by a second channel matrix, and transmitting at least onespectrally modified signal variant of the OFDM data signal comprisespre-coding a signal component of the OFDM data signal, the pre-codingbeing performed in the frequency domain by applying at least twodifferent multiplicative factors to different subcarriers for afrequency-dependent multiplication to the respective signal component ofthe OFDM data signal; and selecting the at least two multiplicativefactors in such a way so as to construct a further channel matrix basedon the second channel matrix, wherein the further channel matrixcomprises a rank that is higher than a rank of the first channel matrix.2. The method according to claim 1, wherein the data signal comprises aplurality of signal components, and wherein the second transmission ofeach signal component of the data signal is spectrally modified toobtain a respective spectrally modified signal variant of the datasignal.
 3. The method according to claim 1, wherein the at least twomultiplicative factors are selected in order to maximize a rank of thefurther channel matrix.
 4. The method according to claim 1, wherein oneof the following transmits spectrally modified signal components: onetransmission antenna port of the plurality of transmission antennaports, all transmission antenna ports of the plurality of transmissionantenna ports, any number of transmission antenna ports between one andall of the plurality of transmission antenna ports.
 5. The methodaccording to claim 1, wherein the spectrally modified signal variants ofthe data signal are different from each other.
 6. The method accordingto claim 1, wherein the second transmission is initiated upon receipt ofan information that the transmitted data signal is not capable of beingcorrectly decoded based on the first transmission.
 7. The methodaccording to claim 1, further comprising: performing a thirdtransmission of the data signal at a time later than the secondtransmission, the third transmission comprising transmitting the atleast one spectrally modified signal variant of the data signal.
 8. Themethod according to claim 7, wherein the third transmission is initiatedupon receipt of an information that the transmitted data signal is notcapable of being correctly decoded based on the first and secondtransmissions.
 9. The method according to claim 8, wherein the at leastone spectrally modified signal variant of the third transmission isobtained by frequency shifting the at least one spectrally modifiedsignal variant of the second transmission.
 10. The method according toclaim 1, wherein the transmission unit comprises a plurality of storedsets of multiplicative factors, wherein each set comprises informationon at least one spectrally modified signal variant of a second or thirdtransmission, the method further comprising: arbitrarily selecting oneof the plurality of stored sets for performing a second or thirdtransmission of the data signal.
 11. The method according to claim 1,wherein the transmission unit comprises a plurality of stored sets ofmultiplicative factors, wherein each set comprises information on atleast one spectrally modified signal variant of a second or thirdtransmission, the method further comprising: specifically selecting oneparticular set out of the plurality of stored sets for performing thesecond or third transmission.
 12. A transmission unit to transmit a datasignal, comprising: a plurality of transmission antenna ports; a firsttransmission device configured to perform a first transmission of a datasignal, the first transmission comprising transmitting the data signalby each one of the plurality of transmission antenna ports; a secondtransmission device configured to perform a second transmission of thedata signal at a time later than the first transmission, the secondtransmission comprising transmitting at least one spectrally modifiedsignal variant of the data signal by at least one transmission antennaport of the plurality of transmission antenna ports, whereintransmitting at least one spectrally modified signal variant of the datasignal comprises pre-coding a signal component of the data signal, thepre-coding being performed in the frequency domain by applying afrequency-dependent multiplication to the respective signal component ofthe data signal; wherein the data signal is an orthogonal frequencydivision multiplex (OFDM) signal and applying a frequency-dependentmultiplication comprises applying at least two different multiplicativefactors to different subcarriers of the orthogonal frequency divisionmultiplex signal; wherein the first transmission is represented by afirst channel matrix and the second transmission is represented by asecond channel matrix, and a selector configured to select the at leasttwo multiplicative factors to construct a further channel matrix basedon the second channel matrix, the further channel matrix comprising arank that is higher than a rank of the first channel matrix.
 13. Thetransmission unit according to claim 12, further comprising: a thirdtransmission device configured to perform a third transmission of thedata signal at a time later than the second transmission, the thirdtransmission comprising transmitting at least one spectrally modifiedsignal variant of the data signal.
 14. The transmission unit accordingto claim 12, wherein the second transmission is configured to transmitat least one spectrally modified signal variant of the data signal by asubset of the plurality of transmission antenna ports, wherein thesubset includes a number of transmission antenna ports in a range from 1to N−1, wherein N is the total number of transmission antenna ports. 15.The transmission unit according to claim 12, wherein the secondtransmission device is configured to perform the second transmission ofthe data signal regardless of whether information is received aboutwhether the transmitted data signal is decoded correctly based on thefirst transmission.
 16. A method for transmitting a data signal by atransmission unit of a wireless multiple-input/multiple-output (MIMO)communication system, the transmission unit comprising a plurality oftransmission antenna ports, the method comprising: performing a firsttransmission of a data signal, the first transmission comprisingtransmitting the data signal by each one of the plurality oftransmission antenna ports; performing a second transmission of the datasignal at a time later than the first transmission, the secondtransmission comprising transmitting at least one spectrally modifiedsignal variant of the data signal by at least one transmission antennaport of the plurality of transmission antenna ports, whereintransmitting at least one spectrally modified signal variant of the datasignal comprises pre-coding a signal component of the data signal, thepre-coding being performed in the frequency domain by applying afrequency-dependent multiplication to the respective signal component ofthe data signal, wherein the transmission unit comprises a plurality ofstored sets of multiplicative factors, wherein each set comprisesinformation on at least one spectrally modified signal variant of asecond or third transmission, the method further comprising: arbitrarilyselecting one of the plurality of stored sets for performing a second orthird transmission of the data signal.
 17. A transmission unit totransmit a data signal, comprising: a plurality of transmission antennaports; a first transmission device configured to perform a firsttransmission of a data signal, the first transmission comprisingtransmitting the data signal by each one of the plurality oftransmission antenna ports; a second transmission device configured toperform a second transmission of the data signal at a time later thanthe first transmission, the second transmission comprising transmittingat least one spectrally modified signal variant of the data signal by atleast one transmission antenna port of the plurality of transmissionantenna ports, wherein transmitting at least one spectrally modifiedsignal variant of the data signal comprises pre-coding a signalcomponent of the data signal, the pre-coding being performed in thefrequency domain by applying a frequency-dependent multiplication to therespective signal component of the data signal, wherein the transmissionunit comprises a plurality of stored sets of multiplicative factors,wherein each set comprises information on at least one spectrallymodified signal variant of a second or third transmission, the methodfurther comprising: arbitrarily selecting one of the plurality of storedsets for performing a second or third transmission of the data signal.18. The method according to claim 1, wherein selecting the at least twomultiplicative factors further comprises intentionally selecting themultiplicative factors to force the constraint relating to the rank ofthe further channel matrix.
 19. The method according to claim 1, whereinselecting the at least two multiplicative factors further comprisesintentionally selecting the multiplicative factors to unavoidably resultin the constraint relating to the rank of the further channel matrix.20. The method according to claim 1, wherein selecting the at least twomultiplicative factors further comprises intentionally selecting themultiplicative factors so that the constraint relating to the rank ofthe further channel matrix is forced by the selecting step.
 21. Thetransmission unit according to claim 12, wherein the selector is furtherconfigured to select the at least two multiplicative factorsintentionally to force the constraint relating to the rank of thefurther channel matrix.
 22. The transmission unit according to claim 12,wherein the selector is further configured to select the at least twomultiplicative factors intentionally to unavoidably result in theconstraint relating to the rank of the further channel matrix.
 23. Thetransmission unit according to claim 12, wherein the selector is furtherconfigured to select the at least two multiplicative factorsintentionally so that the constraint relating to the rank of the furtherchannel matrix is forced by the selecting step.